Among batteries in practical use, a lithium ion secondary battery shows the highest energetic density; and it is commonly used for a small electronics. Further, in addition to a compact application, an application for cars is also expected. Among them, a longer-life and further improvement for safety of a lithium ion secondary battery are demanded.
As a lithium ion secondary battery, in general, a polyolefin series organic separator, such as polyethylene, polypropylene, etc., is used in order to prevent a short circuit between positive electrode and negative electrode. Polyolefin series organic separator has a physical property, which melts at 200° C. or less. Thus, when battery has a high temperature due to its inner or outer stimulation, the organic separator may shrink or melt, causing volume change of an organic separator. As a result, an explosion and the like may occur due to short circuits of positive and negative electrodes, an electrical energy emission and the like.
In order to solve a problem, caused by the use of such polyolefin series organic separator, a layer comprising non-conductive particles such as inorganic particles is proposed to stack on polyolefin series organic separator or on electrode (positive electrode or negative electrode). Further, in order to prevent thermal runaway caused by unusual reaction of a battery, a porous membrane, including polymer particles which melt by heat or which degree of swelling to electrolytic solution increase by heat, is proposed. In case when a temperature of a secondary battery abnormally rises, such as by a short circuit, fine holes in a porous membrane are obstructed by melting and swelling the polymer particles; and thus, ion conduction between electrodes is prevented, electrode is interrupted and it is believed to have a function to prevent further rise in temperature (a shutdown function).
For instance, Patent Article 1 describes a porous membrane comprising heat resistant resin fine particles and organic fine particles having said shutdown function, in order to improve its safety. Further, it is described to use an ethylene vinyl acetate polymer as binder for a porous membrane.
Patent Article 2 describes an improvement in powder falls of non-conductive particles from a porous membrane, by using water-dispersible acrylic polymer particles having a hydrophilic group, such as sulfonic acid. In addition, the article describes a strong and flexible porous membrane can be provided by further including a crosslinkable group in the water-dispersible acrylic polymer particles.
Patent Article 3 describes a porous membrane, in which alumina or titania is used as an inorganic filler, and polyvinyl alcohol or styrene-butadiene copolymer is used as a binder.